1. Field of the Invention
The present invention relates to a webbing retractor provided with a pretensioner tensing a webbing at the time of a sudden or rapid deceleration of a vehicle and a force limiter preventing a tension of a predetermined value or more from being applied to the webbing.
2. Description of the Related Art
There is a webbing retractor installed into a vehicle provided with a pretensioner for winding and tensioning a webbing when the deceleration of a predetermined value or more occurs to the vehicle and giving constraints to an occupant of a vehicle (see Japanese Patent Application Laid-Open No. 4-92748). FIG. 26 shows a webbing retractor 500 provided with the above-stated pretensioner.
In the webbing retractor 500, a cylindrical spool 506 around which a webbing 504 is wound is rotatably supported by a frame 502.
A pretension shaft 508 integrally provided on one end of the spool 506 is inserted with play into a drum 510. A drum 510 is molded to be generally cylindrical as a whole such that two generally semi-annular clamping portions 512 are connected through a generally S-curved connection portion 514. An intermediate portion of a wire 518 whose one end is connected to a drive plate 516 is wound around the outer periphery of the drum 510. An insertion hole 520, into which the drum 510 is inserted, is formed at the center of the drive plate 516. An engagement part 522 protruding from the drum 510 is housed in a wider diameter portion 520A formed in the insertion hole 520, whereby the engagement part 522 is rotated integrally with the drum 510.
The other end of the wire 518 is connected to a piston (not shown) within a cylinder 524. When the deceleration of the vehicle becomes a predetermined value or more, a gas generator operates and the piston is rapidly pulled into the cylinder 524.
The other end of the spool 506 is provided with a locking part (not shown). If a deceleration of a predetermined value or more is detected, then the locking part locks the rotation of the spool 506 in the direction of unwinding the webbing 504 and allows the spool 506 only to be rotated in the winding direction of the webbing 504.
Thus, when a deceleration of a predetermined value or more occurs in the vehicle, the wire 518 is pulled. Due to this, the wire 518 is tightly wound on the drum 510, the connection part 514 is deformed and the clamping portions 512 become closer to the connection portion 514. The clamping portions 512 clamp the pretension shaft 508 and the drum 510 becomes integral with the spool 506. If the wire 518 is further pulled, the spool 506 is rotated by the resultant tension and thus the webbing 504 is wound on the spool 506.
Meanwhile, there has been proposed a webbing retractor provided with a force limiter preventing the tension applied to the webbing from exceeding a predetermined value.
In a webbing retractor provided with a so-called torsion bar type force limiter (see Japanese Patent Application Laid-Open No. 7-47923), a circular torsion bar torsion-deformable in the circumferential direction is inserted into the cylindrical spool in a coaxial manner with the spool. One end of the torsion bar is fixed to the spool. When the vehicle is suddenly decelerated, the other end of the torsion bar is locked by a locking mechanism. Due to the fixation of one end of the torsion bar to the spool, if tension in the unwinding direction acts upon the webbing at the time of a the deceleration of the vehicle, then the torsion bar is torsionally deformed to rotate the spool, thereby preventing the tension load of a predetermined value or more from being applied to the webbing.
In the meantime, the webbing retractor 500 has a structure that at the time of a sudden deceleration of the vehicle, the drum 510 is tightly wound by the wire 518, the clamping parts 512 clamp the pretension shaft 508 and the drum 510 is rotated together with the spool 506. Due to this, while the wire 318 is unwound to a maximum, the tension acting on the wire 518 prevents the spool 506 from being further rotated in the unwinding direction of the webbing 504. Accordingly, it is impossible to sufficiently torsion-deform the torsion bar in the webbing reactor 500 in which the above-stated force limiter is merely installed.